This invention pertains to an apparatus for heat sealing an electron gun mount, including a glass stem wafer, into a neck of a cathode-ray tube having a circular cross-section.
A standard cathode-ray tube (CRT) comprises a faceplate panel with a cathodoluminescent screen, a funnel having a protruding neck, and a mount containing an electron gun adapted to emit a beam of electrons for striking the screen. The mount includes a glass stem wafer on which the gun is mounted with lead wires for the gun electrodes projecting through the wafer. Round portions of glass called "fillets" surround the lead wires at the glass wafer to provide a better seal around the lead wires. The funnel typically is sealed to the faceplate panel in a high-temperature oven using a glass frit before the mount is sealed to the neck of the funnel. After the frit-sealing step, the mount is seated to the neck by a heat-sealing apparatus which applies high-temperature flames to a localized area around the neck where the seal is to be formed.
During this mount-sealing process, the CRT is typically held in a vertical panel-up position by a cradle on a "carrousel". The wafer with the gun mounted thereon is upwardly inserted into the neck by a mount pin or socket. The mount is aligned, so that the gun electrodes are aimed to properly strike the cathodoluminescent screen, and held thereat by the mount socket while heat from burners is applied to the outside of the neck proximate the wafer. The CRT and wafer are rotated about a vertical central axis as they are indexed to various stationary burner locations around the carrousel. At these locations, the burners are positioned around the vertical central axis of the CRT neck so that the neck softens, thins, and then seals to the wafer. Also, excess neck material that is lower than the wafer (cullet) is cut off and falls away from the neck.
Presently, there is a trend toward larger CRTs. This means that the carrousel and the rotating machinery, including two vertical support bars for the CRT, must be relatively large and heavy and, therefore, more expensive. Furthermore, the two vertical support bars interfere with the placement of an electrical resistance oven around the CRT neck for a preferred preheating step before the wafer-sealing operation and an annealing step after sealing. Thus, additional burners are required to perform these steps instead of the resistance oven. This requires additional carrousel locations, and the heating provided by the burners is not as uniform as that provided by the resistance oven.
In order to accomodate larger CRTs, our copending application entitled APPARATUS FOR HEAT SEALING A GUN MOUNT IN A CRT NECK, Ser. No. 648,809, filed on Sept. 10, 1984 and assigned to RCA Corporation, teaches a specific method which achieves the aforementioned mount-sealing operation at a single stationary location and obviates the need for the carrousel and heavy rotating machinery. This method comprises heating completely around a nonrotating CRT neck by providing a vortical pattern of flames around the neck from a plurality of nonrotating burner tips similarly aimed nonradially along directions tangent to points substantially halfway between the outside diameters of the neck and the wafer. The flames are provided, firstly, above the plane of the wafer to cause a reduction in the diameter of the neck; secondly, below the plane of the wafer to cut off excess material from the neck; and thirdly, in the plane of the wafer, i.e., the "seal plane". Consequently, in achieving a successful mount-sealing operation at a single stationary location without the need for the carrousel and heavy rotating machinery, it was necessary to raise the burner tips slightly above the seal plane, then lower the burner tips to a position slightly below the seal, and then raise the tips to the seal plane. Also, after lowering the burner tips, the gas mixture was adjusted to change the flame temperature so as to provide a higher temperature at the latter two positions than at the neck-in position, above the seal plane. The present invention provides an apparatus which does not have to be raised or lowered during the neck-in, cut-off and sealing operations.